The invention relates to an electric lamp comprising
a lamp vessel in which a light source is arranged;
a lamp cap connected to the lamp vessel and provided with electrical contacts;
current supply conductors connected to the light source and to respective contacts of the lamp cap;
which lamp cap comprises a metal shell having a cylindrical portion connected to a conical portion, the cylindrical portion enclosing an angle with the conical portion.
The invention further relates to a lamp cap for use in the lamp.
Such a lamp is generally known and is described in, for example, U.S. Pat. No. 5,187,407. The lamp cap of the lamp may be of the Edison type. The lamp cap comprises the metal shell, which forms a first electrical contact, and a base portion of insulating material, for example of glass, such as vitrite glass, which base portion supports a metal disc which forms a second electrical contact.
The cylindrical shell portion and the metal disc of the known lamp are interconnected by the insulating material. The cylindrical portion is connected to a threaded portion, via a bend and the conical portion of the shell, and encloses an angle with the conical portion. By the angle enclosed by the cylindrical portion and the conical portion is meant the angle enclosed by the relevant generating lines. When the lamp is placed in a lampholder, a bottom contact member of the lampholder comes into contact with the second electrical contact, which as a result is exposed to a compression force in axial direction, i.e. parallel to the direction from the second electrical contact to the light source. The axial compression force is subject to a lower limit so as to realize a good contacting between the bottom contact member and the second electrical contact throughout lamp life.
It may happen during use of the known lamp that the lower limit of the axial compression force is amply exceeded. This increases the risk of the conical portion of the shell becoming deformed, for example in that it is made to buckle up over the insulating material, in which case the lamp cap is damaged and the contacting between the base contact member and the second electrical contact is absent, so that lamp life is prematurely terminated.
It is a disadvantage of the known lamp that its cost price is comparatively high because comparatively much material is required for the shell if the latter is to be made sufficiently strong, i.e. for withstanding axial compression forces at or somewhat above the lower limit.
It is an object of the invention to provide an electric lamp of a kind as described in the opening paragraph in which the above disadvantage is counteracted.
This object is achieved in that the conical portion comprises a first portion and a second portion bording the cylindrical portion which enclose respective angles with the cylindrical portion, an angle xcex2 between the second portion and the cylindrical portion being smaller than an angle xcex1 between the first portion and the cylindrical portion. The risk of a forced inward buckling deformation of the cylindrical portion is considerably reduced thereby. Instead of buckling inwards, the cylindrical portion can bolster itself against the insulating body, whereby a comparatively high axial strength of the shell is achieved.
Experiments have shown that the risk of an inward buckling of the cylindrical portion substantially does not exist when the angle xcex2 between the second portion and the cylindrical portion is no more than 35xc2x0. The shape of the conical portion of the shell, with the angle xcex2 of at most 35xc2x0, renders it possible to realize a thinner shell compared with the present state of the art, without a substantial change in the strength in axial direction of the shell occurring.
In a favorable embodiment, the angle xcex2 between the second portion and the cylindrical portion is at most 5xc2x0. The production of the shell is comparatively simple then because the second part can be folded against the cylindrical part, which means that the production equipment does not need an accurate adjustment. The second portion will spring back slightly owing to the elasticity of the material, but the angle xcex2 enclosed by the second portion and the cylindrical portion after that will then be 5xc2x0 at the most.
It is favorable for lamp caps of the Edison type when the shell has not only the adapted shape of the conical portion but also a threaded portion which is provided with, for example, two or more longitudinal grooves, i.e. elongate depressions extending in axial direction towards the cylindrical portion. These longitudinal grooves give the shell a certain degree of stiffness in axial direction and reinforce the threaded portion of the shell. The use of a metal, for example aluminum, of a smaller thickness is then possible for, for example, an E27 lamp cap without an accompanying substantial change in strength in the axial direction of the shell.